The region is underlain by volcanic and sedimentary rocks of the late Triassic to early Jurassic Nicola Formation. These rocks have been intruded by Tertiary volcanics, dikes, sills and around Rey Lake by granitic rocks of variable composition.
Within the local region, the Nicola rocks are fault bounded and are believed to occupy a graben structure. Intrusive rocks of the Guichon Batholith (which host the famous Highland Valley porphyry copper mineral deposits) lie to the west of the Nicola rocks. On the east side of the graben, Jurassic aged intrusives of the Nicola Batholith occur. Intrusive rocks of the Tertiary, Iron Mask Batholith about the Nicola Graben on its’ northeast side.
Locally, outcropping bedrock is scarce, particularly around Rey Lake and extending about 2 km southwards, about halfway to Swakum Mtn. Within this area overburden depth reaches as much as 300 feet on the north side of Rey Lake but is commonly only a few meters on the southern side where sporadic outcrop occurs. Bedrock is more commonly exposed on the upper, north and west flanks of Swakum Mountain where limestones, shales and volcanic rocks have been subjected to thermal alteration, locally reaching garnet skarn assemblages (i.e. Lucky Mike showings).
Intrusive quartz monzonite rocks of the Rey Lake Pluton, have been mapped beneath Rey Lake and extending in a "finger" southwards for about 1 km. The extent of the intrusive has been largely inferred from widely spaced percussion and diamond drill holes, dating from the 1972-1976 episodes of exploration.
The area around Swakum Mountain consists of folded Upper Triassic Nicola Group volcanic rocks with interbedded sedimentary units. These rocks are intruded by large north trending felsic to intermediate intrusions (batholiths) east and west of the mountain. Nicola Group rocks on the mountain strike north to northeast with generally steep dips. For a large part they consist of andesitic flows and tuffs, agglomerates, and occasional basalts and rhyolites.
A break occurs in the volcanic stratigraphy and is comprised of a mixed volcanic-sedimentary unit consisting of a thick sequence of felsic volcanic flows, lithic and crystal tuffs, limy sediments and a prominent limestone. This unit has a northeast strike and crosses the mountain for a 2.5 kilometre strike length. The unit has been historically used as a marker horizon in interpreting a large, asymmetrical, south plunging anticline with its north trending axis near Swakum Mountain summit. Narrow quartz porphyry dykes locally intrude the Nicola Group sequence.
To the east of this marker unit are a thick, unconformable wedge of immature sediments, predominantly coarse polymictic conglomerates (fan-type) and grits with minor cherty units. Most of the old workings on the mountain occur in close proximity to or within this volcanic-sedimentary unit. The Swakum Mountain deposits consist of polymetallic skarn-type mineralization, lead-zinc-silver bearing quartz veins and replacements, and polymetallic quartz veins.